Membrane separation processes for separation of liquids and substances dissolved in liquids and for gases have become very important in recent years and are replacing more and more conventional separation technologies.
The performance of the separations achieved is depending in the first place on the membrane material and the membrane thickness.
Since the flux rate of a membrane is inverse proportional to the thickness of the membrane, the membrane has to be very thin in order to achieve flux rates high enough for practical applications. Large surface areas of very thin membranes are difficult to produce and to handle, therefore they have to be supported by a porous media providing mechanical strength without reducing noticeably the membrane performance ("composite membrane").
Another method to produce high-flux membranes has been invented by Loeb et al, US-A-3,133,132. This method result sin asymmetrical membranes having a very thin tight surface layer supported by a spongy sub-structure of the same polymer material. This type of membranes is used today in large-scale separation systems for liquid media, e.g. in connection with water desalination by reverse osmosis. The original asymmetrical Loeb membranes were made from cellulose acetates and cellulose acetobutyrates and show a relatively small number of defects like so called pinholes. The Loeb process has also been applied to other polymers. However, it has been found that large and costly experimental programs had to be carried out before acceptable asymmetrical membranes could be obtained from non-cellulosic polymers. In addition, these membranes have quite generally more pinholes and other penetrating surface defects than those prepared from cellulose acetate.
Despite the larger number of pinholes, asymmetrical membranes made from non-cellulosic polymers have been applied successfully to separations in liquid systems, since the relatively high viscosity and high cohesive properties of liquids, as well as adsorption on and swelling of the membranes, limit the negative effects of pinholes.
For gas separations, however, pinholes in membranes present a much more severe problem since the gas transport through these holes is five to six orders of magnitude (10.sup.5 -10.sup.6) higher than the transport through the membrane material. This is due to the low absorption and the very low viscosity and cohesive properties of gases.
A very important difference between liquid and gases is also the generally much lower solubility of the gases in the membrane polymer.
MONSANTO company, St. Louis, Mo., USA, describes US-A-4,230,463 multicomponent membranes for gas separations comprising a coating in contact with a porous separation membrane wherein the separation properties of the multicomponent, or composite, membrane is principally determined by the porous separation membrane as opposed to the material of the coating. This is achieved by choosing a coating material, e.g. polysiloxanes which exhibits less resistance to permeate gas flow than the porous support material, e.g. polysulfone. The support material, however, has a better separation factor and therefore determines the efficiency of the gas separation. Since the support has also a higher resistance to the gas flow, it practically also determines the flux rate. The beneficial effect of the coating is therefore mainly the occlusion of the pinholes and the protection of the polysulfone membrane against damages during handling and assembly in modules.
Large-scale gas separation systems based on asymmetrical hollow fiber composite membranes as described in US-A-4,230,463, have been developed by MONSANTO and commercially applied to different gas separations.
The company DOW CHEMICALS has developed symmetrical hollow fiber membranes based preferentially on polymethylpentene. Due to the extremely high gas permeability of this polymer, flux rates high enough for practical applications are obtained despite the symmetrical nature of the membrane. Since the thickness of this unsupported symmetrical membrane had to be reduced as much as possible while preventing a membrane collapse due to the pressure difference applied to the hollow fibers during operation of the membrane modules, hollow fibers with an extremely small diameter, i.e. less than a human hair, resulted from DOW's development.
DOW is commercialising their gas separation membrane in form of their GENERON modules and systems for air separation used for production of 90-98% nitrogen gas or oxygen enriched air.